1. Field of the Invention
The invention relates to a photoluminescence quenching device (PQD) which can be driven in an emissive mode to convert signal voltages into light and in a re-emissive mode to suppress a photoluminescent emission, a display based on photoluminescence quenching devices (PQD) and a process to convert signal voltages into optical picture information.
2. Description of the Related Art
Flat displays based on organic light emitting diodes (OLEDs) glow brightly and have a wide viewing angle. Self-emissive OLED-displays do not require background lighting and can be implemented such that the display energetically functions favorably in conditions with low or medium amounts of ambient light.
Under conditions with large amounts of ambient light, e.g. in direct sunlight, however, in self-emissive OLED-displays, a relatively large amount of power is required to attain the appropriate brightness. Furthermore, the required currents needed to control the emissive components are accordingly high. Therefore, in this regard, reflective and re-emissive technologies like liquid crystal displays are superior to self-emissive OLED-displays. The disadvantage of reflective and re-emissive technologies is, however, that they require a backlight for operation in conditions with little ambient light, which in turn over proportionately increases energy consumption and the form parameters.
Devices which are based on organic light emitting diodes and which operate in an emissive mode as well as a re-emissive mode are disclosed in DE 100 42 974 A1. Such a photoluminescence quenching device (PQD) has a structure which is similar to an organic light emitting diode and can operate in both the self-emissive mode, i.e. without ambient light, as well as in the re-emissive mode. In the re-emissive mode, the intensity of the photoluminescent light is controlled by applying a voltage in the inverse direction of the PQD. Appropriately selecting contact and emitter materials enables the device to operate in the emissive mode as well as in the re-emissive mode. A further advantage is that the device does not require a backlight. A condition for the re-emissive operation is that enough ambient light can be absorbed from the surroundings. It is also necessary to be able to apply positive and negative gate voltages to the display element.
In their basic structure, photoluminescence quenching devices consist of a transparent and conducting contact, an emitter layer, and an opposing contact. To regulate the brightness of the display element the metal contact (i.e., opposing contact) is positively charged in relation to the transparent contact and this corresponds to the inverse direction (re-emissive operation). This structure does, however, have limits in regards to efficiency while in re-emissive operation, since the injection barrier is also finite in the inverse direction and consequently noticeable dark currents flow. These dark currents diminish the energy balance, especially when the display is operated with low brightness. Furthermore, the metal contact must be a base metal like aluminium or calcium, which require a complex encapsulation to protect the contact from corrosion.
It is known from U.S. Pat. Nos. 5,294,810 and 6,097,147, that multiple layer structures can improve the energy balance. These structures are optimized for optimal performance in emissive operation of the light emitting diode, i.e. for optimal injection of charge carriers. Likewise, it is known that the application of inorganic materials in organic light emitting diodes improves the injection of electrons during emissive operation.
Moreover, EP 1 083 612 describes the application of fluorides and oxides of alkali metals and alkaline-earth metals to improve electron injection. All known multi layer systems for organic light emitting diodes, however, only realize a favorable injection of electrons in the emissive mode. With the currently known devices it is not possible to reduce the electron injection for re-emissive operation of a light emitting diode.